WASHINGTON (CNN) -- The Supreme Court on Monday tossed out a provision in Arizona's voter registration law that required proof of citizenship.

The 7-2 majority said the state's voter-approved Proposition 200 interfered with federal law designed to make voter registration easier.

The state called the provision a "sensible precaution" to prevent voter fraud. Civil rights group countered that it added an unconstitutional and burdensome layer of paperwork for tens of thousands of citizens.

Justice Antonin Scalia said the National Voter Registration Act of 1993 "forbids states to demand an applicant submit additional information beyond that required by the federal form."

But in a nod to state authority, Scalia said the federal law "does not prevent states from denying registration based on any information in their possession establishing the applicant's eligibility."

The appeal was a classic federalism dispute, on the often delicate line between conflict and cooperation between state and federal governments over enforcing voting procedures. During last year's election, there were numerous court challenges to state voter identification laws at the polls. The current fight has produced a range of states, lawmakers and advocacy groups on both sides on the gateway issue of registration. The Obama Justice Department opposed the Arizona law, which went beyond what other states have done to ensure integrity in the registration system.

Retired justice Sandra Day O'Connor, an Arizona native, was among those who attended the spirited April oral arguments.

Justice Anthony Kennedy a year ago blocked the Arizona law from being enforced, while the high court decided internally whether to accept pending appeals for review. The ballot measure was passed in 2004 and has been lingering in the federal courts ever since.

The Constitution's Article I says "the times, places, and manners of holding elections for senators and representatives shall be prescribed in each state by the legislature." But Congress is also given the power "to make or alter such regulations."

Federal lawmakers did just that, passing the National Voter Registration Act two decades ago, which has since been called the Motor Voter Law, designed to streamline election participation.

It requires states to have any application for a driver's license treated also as a voter registration -- the "motor voter." And it requires states to "accept and use" mail-in and in-person applications. A federal Election Assistance Commission was created to produce a nationally uniform voter application form, which states must use. Any extra state instructions, or "add-ons," must be approved by the commission.

The question was whether certain extra instructions are permitted, and just how the federal from must be respected in the first place.